Antigens vs Immunogens
The terms antigen and immunogen are often used interchangeably, and for most purposes, the difference is irrelevant. But to be precise, they describe two types of interactions between a molecule and the immune system.

An immunogen refers to a molecule that is capable of eliciting an immune response by an organism’s immune system, whereas an antigen refers to a molecule that is capable of binding to the product of that immune response. So, an immunogen is necessarily an antigen, but an antigen may not necessarily be an immunogen.

Throughout this site, the term antigen will be used since it refers directly to the molecule that binds to the product of the immune response – the antibody.